The invention relates to a sleeve with a multiple layer structure for printing presses with king rolls designed as air cylinders, in particular to a sleeve for the flexographic printing process, with an inner tube of reversibly expandable plastic material, whose internal diameter is smaller than the king roll diameter, with an elastic compressible intermediate layer accomodating the radial expansion of the inner tube during mounting or dismounting of the sleeve, and with an outer layer. The invention relates also to a method for manufacturing sleeves of this type with the steps of manufacture of a reversibly expandable inner tube, applying compressible intermediate layer on the outer surface of the inner tube and applying a single or multiple ply transition layer of a low density material that can be cast or foamed, and/or applying of an outer layer.
In the printing industry, in particular in the flexographic printing process, the state of the art technology has for years included working with sleeves, which carry the printing layer, the printing block or plate for the later printing process, and which can be pushed on to the significantly more expensive king rolls, supported in bearings in the printing presses, and configured as air cylinders, mostly of metal or with a circumferential layer of metal. The installation and de-installation of the sleeve takes place using the air cushion principle, in which compressed air exits via at least one radial hole on the outer surface of the king roll, with which air the inner tube of the sleeve is reversibly expanded during the installation or de-installation, so that it can be pushed axially on to the king roll in its expanded state, or can be pulled off the same, with low expenditure of force. When the compressed air supply is switched off the inner tube of the sleeve contracts again and the sleeve sits on the outer surface of the king roll with a shrink fit, secured against rotation, as is known art from e.g. EP 196 443 B1. The reversible, expandable inner tube consists mostly of a relatively thin-walled plastic tube laminated with fibre inlays, which is directly surrounded by a thicker walled layer of elastic compressible material, in particular such as a soft foam, which makes possible or allows the expansion of the inner tube during installation or de-installation of the sleeve.
Onto the elastic compressible layer can directly be applied an outer layer, which comprises a surface suitable for reception of the printing plates, printing platens or printing layer, or between the soft foam layer and the outer layer is located an incompressible intermediate layer, for example of a cast mass of low density or a rigid foam, as a result of which the diameter of the sleeve in the radial direction can be increased by up to 100 mm, without the weight of the sleeve increasing significantly. The various print patterns can be generated by means of the wall thickness of the sleeves used. However for printing sleeves the variation of wall thickness in the radial direction is limited, since with increasing thickness of the sleeves the deviations in diameter and concentricity increase and an exact print can no longer be guaranteed. For these reasons the printing industry has therefore changed its practice to inserting adapter sleeves between the pneumatic cylinders and the sleeves, which are themselves fitted with a compressible layer and, using the air cushion principle, can be axially pushed on to the pneumatic cylinder. The adapter sleeves in turn are fitted on their outer surface with an air feed for the compressed air, so as then to be able to install the sleeve bearing the print motif or the printing plate onto the pre-assembled unit of pneumatic cylinder and adapter sleeve.
EP 0 753 416 A1 discloses to firstly assemble the adapter sleeve and the sleeve carrying the print motif, and then to install the composite structure of adapter sleeve and print sleeve onto the king roll of the printing press, designed as an air cylinder, using the air cushion principle.
From DE 195 45 597 A1 a sleeve of like type is known art, whose outer layer consists of a metal tube that can directly be engraved. The space between the expandable inner tube and the elastic compressible layer, and also the outer tube, is filled with a foam of PUR, BS, UF, PF, PVC or PE.
When printing with sleeves, which for purposes of installation comprise at least one compressible layer between the inner tube and the outer layer, or are assembled on adapter sleeves, which comprise a corresponding compressible layer, axial lines often appear in the print image, for which up to the present time various causes have been attributed. One cause for these lines, identified by expert groups, as vibration stripes, is seen to be the tapes with which the printing plates are installed on the outer layer of the sleeve, and which should prevent relative movements between the plates and the sleeves. In particular with the use of thick and soft tapes it can be necessary for a good colour transfer to increase the feed pressure between the king roll and the impression cylinder, as a result of which the gears of the king roll and the impression cylinder can engage too fiercely, such that axial gear stripes or chatter marks can appear in the print image. Other causes for axial stripes or vibration lines are seen to be worn-out bearings and the vibrations that are thereby generated, as well as an unfavourable drive layout that generates vibration harmonics during the printing process.
In modern printing presses the running performance of the presses, in particular the rotational speed of the king rolls and the impression cylinders is continuously increasing, and the printing press manufacturers, by means of new types of drives, lighter materials for the king rolls used, and improved bearing arrangements, can increase by any amount rotational speeds that seemingly can be controlled. With increasing rotational speeds of the king rolls and impression cylinders it is seen however that the problem of vibration lines or vibration stripes increases more than proportionally. In particular vibration lines then appear when working with print sleeves or adapter sleeves for large pattern repeat lengths, which comprise compressible layers for installation using the air cushion principle.